Atrial fibrillation is a common cardiac arrhythmia involving the two upper chambers (atria) of the heart. In atrial fibrillation, disorganized electrical impulses that originate in the atria and pulmonary veins overwhelm the normal electrical impulses generated by the sinoatrial node, leading to conduction of irregular impulses to the ventricles that generate the heartbeat. Atrial fibrillation can result in poor contraction of the atria that can cause blood to recirculate in the atria and form clots. Thus, individuals with atrial fibrillation have a significantly increased risk of stroke. Atrial fibrillation can also lead to congestive heart failure or, in extreme cases, death.
Common treatments for atrial fibrillation include medications or synchronized electrical cardioversion that convert atrial fibrillation to a normal heart rhythm. Surgical-based therapies have also been developed for individuals who are unresponsive to or suffer serious side effects from more conventional treatments. The surgical techniques include making incisions in the right and left atria to block propagation of the abnormal electrical impulse around the atrial-chamber.
Catheter-based contact ablation techniques have evolved as a minimally invasive alternative to surgical-based techniques, and also as an alternative for individuals who are unresponsive to or suffer serious side effects from more conventional treatments (e.g., medications). Contact ablation techniques involve the ablation of groups of cells near the pulmonary veins where atrial fibrillation is believed to originate, or the creation of extensive lesions to break down the electrical pathways from the pulmonary veins located on the posterior wall of the left atrium. Methods of energy delivery include radiofrequency, microwave, cryothermy, laser, and high intensity ultrasound. The contacting probe is placed into the heart via a catheter that enters veins in the groin or neck and is routed to the heart, thus negating the need for an incision in the heart wall from the outside. The probe is then placed in contact with the posterior wall of the left atrium and energized to locally ablate the tissue and electrically isolate the pulmonary veins from the left atrium. The advantages of catheter-based contact ablation techniques have been recognized to include a minimally invasive surgical access, thus reducing risks of infection, and reduced recuperation times.
Where complete electrical isolation is desired, the objective of the contact ablation technique is to form a continuous “ablation line” or “isolation line” of ablated tissue between the left atrium and the pulmonary veins. Two different approaches for achieving an isolation line have been developed: point contact ablation where the energy delivery is from a head end of the contacting probe generally in line with a longitudinal axis of the contacting probe; and linear contact ablation where the energy delivery is from a side of the contacting probe and generally transverse to the longitudinal axis of the contacting probe.
A concern with catheter-based contact ablation techniques is the post-operative recurrence of atrial fibrillation, believed to be caused by electrical reconnection of pulmonary veins across the isolation line. The sites along the isolation line where this type of electrical reconnection occurs are referred to as “isolation gaps” or simply “gaps.” Gaps can occur due to suboptimal catheter contact force during ablation for either point contact ablation or linear contact ablation techniques. The left anterior wall is often a difficult area to achieve stable contact during pulmonary vein isolation resulting in higher incidence of local isolation gaps.
One approach to identifying or predicting possible isolation gaps has been to make electrical continuity measurements across the isolation line after the isolation line has been created. While this approach may work in some cases for linear contact ablation techniques, it is generally not effective for point contact ablation techniques because it requires too much time and too many continuity measurements in order to establish a relatively high confidence in the ability to predict whether there will or will not be isolation gaps as a result of incomplete lesion formations during the ablation process of creating the isolation. In addition, it has been found that intra-operative continuity measurements of the isolation line may not be an accurate predictor of the recurrence of atrial fibrillation as the tissue properties of the lesion just after ablation can change over time and may not be representative of the final lesions associated with the isolation line.
The predictability of lesion formation in the context of point contact ablation techniques has been enhanced with the advent of force sensing ablation catheters. The ability to incorporate the contact forces utilized in point-to-point ablation procedures has lead to new systems and processes directed to the prediction of ablation size. United States Patent Application Publication No. 2010/0298826 to Leo et al. (Leo), assigned to the assignee of the instant application, discloses the use of a force-time integral for real time estimation of lesion size in catheter-based ablation systems.
Further improvements in the application of force sensing catheter-based contact ablation devices and methods to mitigate the occurrence of electrical reconnection across isolation lines after ablation treatments for atrial fibrillation would be a useful and welcomed development.